This invention relates to the productions of electrical cables in general and more particularly to an improved method and apparatus for SZ-twisting electrical cables.
Various methods and apparatus have been developed recently which can be used for twisting elements with a twist direction which changes from section to section. This type of twisting which is widely used in twisting electrical cables is known as SZ-twisting. Such a technique has the advantage that the twisting elements run off from stationary frames or supply containers and can be wound on take-up drums also arranged in a stationary manner. This permits performing several operations in series which were previously performed separately. For example, the insulating of conductors and subsequent twisting or several consecutive twisting stages of the twisting elements and the subsequent spraying of the sheath can be accomplished in the same operation. The origin of the SZ-twisting technique was a method in which conductors of communications cables running from stationary reels were twisted by means of a perforated or twisting disk arranged ahead of a twisting nipple the disk executing an oscillating motion relative to the twisting axis. In such a device or method the twisting elements run directly from the conductor supplies through the twisting disk into the twisting nipple. In an arrangement of such nature, the twisting disk can execute only one or at most two revolutions in one direction. (See, for example, German Patent No. 610,650). This known twisting apparatus has not been found acceptable in practice for twisting conductors for use in communications cables because the twisting lays obtainable in one direction are too small. In order to carry-out as the twisting of conductors for electrical cables, methods and apparatus have been developed permitting the twisting elements to be twisted with the same twist direction over a longer section and then subsequently twisted with an opposite direction for an equivalent distance. For example, twisting in one direction over lengths of 5 to 20 meters with the minimum length of lay of about 10 meters is needed. One method which is suited for this purpose causes the elements to be first fed to a first fixed point for forming into a twisted unit. The twisted unit formed from the twisting elements remains in a stretched, torsionable condition between the first fixed point and the second fixed point. It is then twisted in a section-wise manner by a multiple of twisting lays using a twisting device which engages the twisted unit perpendicularly to its axis with positive force transmission. It is quite important that the twisting device be arranged closer to the second fixed point than to the first fixed point and that the rotary motion of the twisting device is changed at intervals which are smaller than three times the time for the passages of a cross-section element of the twisted element through the torsioning section defined by the first and the second fixed points. A device of this nature is disclosed in U.S. Pat. No. 3,593,509. In order to operate such a twisting apparatus with maximum efficiency, it is advisable that the twisting device changes its direction of rotation at intervals. In the process, however, an overtwisting of the twisting elements can occur between the first fixed point and the twisting device. This has an adverse effect on the electrical coupling of the twisted unit, particularly when twisting of conductors to form a unit such as a spiral quad.
In order to prevent overtwisting of conductors which may result in the SZ-twisting of conductors to form spiral quads, a method has been developed in which several twist reversal devices in the form of a perforated rotating disk, by means of which an already twisted material is temporarily untwisted and twisted again, have been used. This type of arrangement has been used in SZ-twisting devices in which a constant length of lay is imparted to the material being twisted within a section with the same twist direction. The perforated disk has a twisting nipple associated with it on both sides and is located behind the respective twisting device and rotates with a speed and direction of rotation which is a function of the length of lay of the twist material in each case, the twist direction and the pull-off velocity. Such is disclosed in British Patent No. 1,144,791.
Another method of avoiding overtwisting comprises providing a torsioning section defined by two twisting points in which, immediately behind the first twisting point, a revolving twisting head is arranged. A similarly revolving twisting head is placed immediately ahead of the second twisting point. The twisting heads always revolve with the same and a constant direction of rotation. Such is disclosed U.S. Pat. No. 3,823,536.
Although these various devices work well for different purposes they do have certain deficiencies. In view of this it is the object of the present invention to provide a SZ-twisting method and apparatus for twisting elements of electric cables at a high production speeds while still permitting an exact twisting of the twisting elements even in the vicinity of the reversal points of the twist direction.